HINT Collect like powers of $x$ from all three terms in your LHS (after figuring out how to write the product of two sums as a single sum); set each coefficient to zero; solve these equations to find the coefficients. If you get bogged down in notation & all that, write out the sums explicitly (sure, up to some order) & try to see a pattern. That's at the very core of the method so you should know it, but I somehow get the feeling that this may be where your problems begin.